1. Field of the Invention
This invention relates to a process for one-sided hot-dip coating a metal sheet.
2. Description of the Prior Art
Steel sheet that has been used for automobiles and electric home appliances has been hot-dip coated on both surfaces insofar as corrosion resistance is one of requisites. From the view-point of the ability to weld and paint the steel sheet, however, it is preferred that it has an uncoated surface. Consequently, demand for one-sided, hot-dip coated steel sheet has been increasing and this also is true for metal sheets other than the steel sheet.
Various methods have therefore been proposed as the production process for one-sided hot-dip coated metal sheet such as those listed below, for example;
(1) A method in which two steel sheets are lapped and only non-lapping surfaces are hot-dip coated (such as disclosed in Japanese Patent Laid-Open No. 125934/1975);
(2) Chemicals such as a silicone resin are applied only to the non-coating surface prior to the hot-dip coating (such as disclosed in Japanese Patent Laid-Open No. 3836/1974);
and
(3) A gas containing oxygen is sprayed only on one surface in order to form in advance a film which reacts with difficulty to a hot-dip coating metal (such as disclosed in Japanese Patent Laid-Open No. 38428/1977).
Though these methods have certainly succeeded in accomplishing the respective intended objects to some extent, none of them are entirely satisfactory, especially with respect to production cost, productivity and the quality of the product.
On the other hand, there has also been proposed a method wherein a bath surface upheaving or raising device (an impeller as a definite example) is disposed inside a hot-dip coating bath in order to upheave or raise and bring the hot-dip coating metal into contact with the lower surface of a steel strip travelling thereover (Japanese Patent Publication No. 25096/1974). This method, however, is not free from the problem of the construction and materials for the upheaving device. Especially when any fluctuation occurs on the bath surface due to carry-out of the hot-dip coating metal, it becomes impossible to form sufficient upheaval in a stable manner. In such a case, a dross (a metal oxide) formed on the surface of the bath of this kind is also raised or upheaved and brought into contact with the steel strip together with the hot-dip coating metal whereby appreciable finish can not be expected on the coated surface. In order to establish sufficient contact of the hot-dip coating metal with the steel sheet, further, the speed of revolution of the impeller, for example, must be enhanced. However, this tends to result in high ripples on the bath surface or occurrence of splashes that often attach to the non-coating side or to operators and do them an injury.